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 A209401 Number of noncommutative rings with n elements. 4
 0, 0, 0, 2, 0, 0, 0, 18, 2, 0, 0, 4, 0, 0, 0, 228, 0, 4, 0, 4, 0, 0, 0, 36, 2, 0, 23, 4, 0, 0, 0 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,4 COMMENTS a(n)=0 if and only if n is squarefree. LINKS Gregory Dresden, Rings of Size Four, demonstrating that a(4)=2. FORMULA a(n) = A027623(n) - A037289(n). EXAMPLE For n=8, there are 52 rings of order 8, 18 of which are noncommutative, so a(8)=18. CROSSREFS Cf. A027623, A037289, A037291. Sequence in context: A283950 A342376 A277443 * A029696 A118887 A057383 Adjacent sequences:  A209398 A209399 A209400 * A209402 A209403 A209404 KEYWORD nonn,nice,hard,more AUTHOR Ben Branman, Mar 26 2012 STATUS approved

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Last modified April 18 02:38 EDT 2021. Contains 343072 sequences. (Running on oeis4.)